Accretion onto a primordial protostar

We present a three-dimensional numerical simulation that resolves the formation process of a Population III star down to a scale of similar to 100 AU. The simulation is initialized on the scale of a dark matter halo of mass similar to 10(6)M(circle dot) that virializes at z similar to 20. It then follows the formation of a fully molecular central core, and traces the accretion from the diffuse dust-free cloud onto the protostellar core for as long as similar to 10(4) yr, at which time the core has grown to similar to 50M(circle dot). We find that the accretion rate starts very high, similar to 0.1M(circle dot) yr(-1), and declines rapidly thereafter approaching a power-law temporal scaling. Asymptotically, at times t greater than or similar to 10(3) yr after core formation, the stellar mass grows approximately as M-* similar or equal to 20M(circle dot)(t/10(3) yr)(0.4). Earlier on, accretion is faster with M-* alpha t(0.75). By extrapolating this growth over the full lifetime of very massive stars, t similar or equal to 3 x 10(6) yr, we obtain the conservative upper limit M-* less than or similar to 500M(circle dot). The actual stellar mass is, however, likely to be significantly smaller than this mass limit due to radiative and mechanical feedback from the protostar. (C) 2003 Elsevier B.V. All rights reserved.